A U.N. chemical weapons expert, wearing a gas mask, holds a plastic bag containing samples from one of the sites of an alleged chemical weapons attack in Syria. Stringer / Reuters

WASHINGTON — The Obama administration knew that a Syrian rebel faction had the ability to make chemical weapons but omitted that knowledge when building its case for a strike on Syria, an explosive new report alleges.

Seymour Hersh reported in the London Review of Books on Sunday that President Obama, while pitching the administration’s case for war, “failed to acknowledge something known to the US intelligence community: that the Syrian army is not the only party in the country’s civil war with access to sarin, the nerve agent that a UN study concluded – without assessing responsibility – had been used in the rocket attack.”

The report, the thrust of which the Obama administration denies, calls into question the narrative that the administration has outlined since an Aug. 21 chemical attack on a Damascus suburb that almost led the United States into an air war with Syria. The march toward war was based on what Obama and his top aides have characterized as conclusive evidence that Bashar al-Assad’s Syrian government had carried out the attack.

The Hersh article is based in part on a four-page secret cable given to a top official at the Defense Intelligence Agency on June 20; it is one of a group of intelligence community documents allegedly stating that jihadi rebel group Jabhat al-Nusra has the ability to make sarin gas. Sarin is the chemical believed to have been used in the Aug. 21 attack in Ghouta that crossed Obama’s “red line” and prompted the administration to push for a strike on Assad’s regime. The story is sourced mainly to intelligence and military officers and consultants.

“When the attack occurred al-Nusra should have been a suspect, but the administration cherry-picked intelligence to justify a strike against Assad,” Hersh writes.

“We were clear with the Washington Post and Mr. Hersh that the intelligence gathered about the 21 August chemical weapons attack indicated that the Assad regime and only the Assad regime could have been responsible,” said Shawn Turner, spokesman for the director of National Intelligence. “Any suggestion that there was an effort to suppress intelligence about a nonexistent alternative explanation is simply false.”

The story was originally intended to run in the Washington Post, a source said. A spokesperson for the Post did not immediately comment on why it didn’t appear there. New Yorker editor David Remnick did not immediately respond to an inquiry Saturday evening as to why the Syria piece did not appear in the New Yorker, which used one of its covers to depict Assad brewing chemical weapons.

The administration has maintained that there is no question of who carried out the Aug. 21 attack. Secretary of State John Kerry said that Assad’s responsibility for it is “undeniable”. “We do not believe that, given the delivery systems, using rockets, that the opposition could have carried out these attacks. We have concluded that the Syrian government in fact carried these out,” Obama said.

Hersh’s article follows a detailed earlier report on the attack in the Wall Street Journal last month, which found that the U.S. failed to analyze intelligence indicating that the Syrian army was preparing a chemical weapons attack until after the attack had happened.

Hersh writes that officials allegedly distorted the information to make it look as though intelligence had been collected in real time when it had not. They did this because, the story says, the U.S. did not have access to crucial intelligence about Syria before the attack happened; the National Security Agency could no longer intercept Assad’s communications. Further, sensors monitored by the National Reconnaissance Office did not pick up on any warheads being loaded with sarin.

Hersh’s conclusion: Obama sought to mislead the nation into war.

“The distortion, [a source] said, reminded him of the 1964 Gulf of Tonkin incident, when the Johnson administration reversed the sequence of National Security Agency intercepts to justify one of the early bombings of North Vietnam,” Hersh writes.

He also raises a more immediate question: “While the Syrian regime continues the process of eliminating its chemical arsenal, the irony is that, after Assad’s stockpile of precursor agents is destroyed, al-Nusra and its Islamist allies could end up as the only faction inside Syria with access to the ingredients that can create sarin, a strategic weapon that would be unlike any other in the war zone.”

Spokespersons for the White House and the State Department declined to comment on the broad outlines of the coming story, which was shared in advance and under embargo with BuzzFeed, Saturday. DNI spokesman Turner told Hersh that no American intelligence agency “assesses that the al-Nusra Front has succeeded in developing a capacity to manufacture sarin.”

Hersh is a legendary, and controversial, figure on the national security beat: His work includes some of the most important scoops of the last four decades, including the massacre at My Lai in Vietnam and the torture scandal at the Abu Ghraib prison in Iraq. He has made claims in other venues that have proven more elusive, including recently charging that official narratives of the death of Osama bin Laden are “one big lie.”